IBM XT 5160

Key Detection Error (Hex FF)
The keyboard sends a key detection error character (hex FF) if
conditions in the
keyboard make it impossible to identify a switch
closure.
Overrun (Hex FF)
An overrun character (hex FF) is placed in the keyboard buffer
and replaces the last code when the buffer capacity has been
exceeded. The code
is sent to the system when it reaches the top
of the buffer queue.
lOl/102-Key Keyboard 4-27